Yes, yes but you need to compromise with people. If I'm going to add "super powers" to the Celts, the Romans have to get something in exchange.
Allowing interpenetration might be a useful counter to allowing 10-20% more Celts? And as I'm sure you know, more figures/units aren't always a blessing.
One other issue is speed. In FoG, Polybian Roman heavy infantry (nearly all of them), move slowly but as drilled troops, can manoeuvre with ease. The Celts, mainly medium infantry, move only slightly faster & can't change formation or direction at all easily. As you can guess, the obvious Roman tactic is to pin the Gauls with your Velites, & bring up the heavies & slaughter them.
I think the Celts should get an improved movement distance. I don't think this is unhistorical, though I agree they should still be cumbersome.
donald